The present invention generally relates to fabrication of semiconductor devices and, more particularly, to a method of fabricating a resin-molded semiconductor package.
Recently, semiconductor devices have become more compact and thinner for increased mounting density. Conventional dual in-line packages (DIP) are being replaced by the small outline J-leaded (SOJ) packages or quad flat packages (QFPs). A new addition to the package types is a thin small outline package (TSOP), which is even smaller than the former two types. As a result of the reduced thickness, the rigidity of the packages has decreased, thus increasing the likelihood that a stage deformation occurs upon ejection of a molded package, and that a crack tends to develop in the package body due to vapor released at the time of the molding process.
Thus, there is a demand for a semiconductor device which is free from the problem of stage deformation and is resistant to thermal stress, such that development of cracks does not occur in the package even when heat is applied at the time of molding.
FIG. 1 is a cross sectional view of a conventional semiconductor device. Referring to FIG. 1, a semiconductor device 11 is configured such that a semiconductor chip 12 is mounted on a stage 13a defined on a lead frame 13 by a die attachment process, wherein the semiconductor chip 12 and a lead 13b are connected electrically with each other by bonding wires 14. Further, by molding the structure that includes the semiconductor chip 12 held on the lead frame 13 in a suitable die, a resin package body 15 is formed to encapsulate the chip 12.
In such a molding process, it is necessary to eject the semiconductor device from the die by a suitable mechanism such as an ejection pin, after the molding is completed. Thereby, it is inevitable that ejection marks 15a-15d are formed on the top and bottom of the package body 15.
FIGS. 2(A) and 2(B) describe how an ejection is performed in a conventional molding process. Referring to FIG. 2(A), the lead frame 13, on which the semiconductor device 12 is mounted by a die attachment process and is wire-bonded with the lead 13b, is placed inside a cavity formed by an upper die 16 and a lower die 17 before a resin mold process is carried out. The upper die 16 is provided with movable pins 18a and 18b so as to bear upon the top surface of the package 15; the lower die 17 is provided with movable pins 19a and 19b so as to bear upon the underside of the package 15.
When the upper die 16 and the lower die 17 are separated from each other after the resin mold process, the movable pins 18a, 18b, 19a and 19b are moved so that the lead frame 13 is ejected out of the upper and lower dies 16 and 17. Therefore, as shown in FIG. 2(B) and FIG. 1, the ejection marks 15a-15d, which are depressions created by the movable pins 18a, 18b, 19a and 19b and have a maximum depth of, for example, 100 .mu.m, are formed on the top surface and the underside of the package 15. The underside of the package is not shown in FIG. 2(B). In such a package structure, it will be noted that the package 15 has a reduced thickness by a maximum of 200 .mu.m at the ejection marks 15a-15d on the top surface and the underside thereof.
The semiconductor device 11 fabricated in the process as described has a drawback in that, as the package 15 gets thinner and, consequently, the semiconductor chip 12 occupies a greater proportion of the space in the package body 15, fluidity of the mold resin becomes poor during the molding process due to restricted space in the mold cavity, thereby causing deformation of the stage 13a and of the wires 14.
It will be noted that the movable pins 18a, 18b, 19a and 19b, which form the ejection marks 15a-15d on the package 15, cause a decrease in the flow of the mold resin, and that a large amount of deformation of the stage or of the wires may result in the wires 14 being exposed through the ejection marks 15a-15d.
Such a deformation of the stage and of the wires may be achieved by increasing the flow of the mold resin, which can in turn be achieved through a reduction in the viscosity of the resin used for the molding. Typically, this is achieved by reducing the percentage of a flexibility enhancer, such as oil, in the mold resin or by reducing the percentage of filler such as silica, in the mold resin.
It is also to be noted that, in the abovedescribed semiconductor device, the resin package body absorbs moisture in the air when it is left exposed at the room temperature, and this moisture turns into vapor when heat is applied during a mounting process of the semiconductor device. The pressure derived from this steam, when applied directly to the package, is great enough for cracks to develop at the corners of the package adjacent to the stage and at the corners of the chip. Consequently, deterioration in moisture resistance results.
Conventional approaches to prevent the package bodies from developing vapor-induced cracks include a process to improve the adhesion between the stage 13a and the mold resin; a process to improve the rigidity (modulus of elasticity) of the resin; and a process to lower the coefficient of moisture absorption of the package 15.
The approach to improve the adhesion includes formation of through holes in the stage 13a as well as formation of dimples or cruciform slits on the stage 13a such that the resin above the stage and the resin below the stage are connected with each other; and a method whereby the material of the mold resin and the lead frame is improved. The reduction of moisture absorption of the resin, on the other hand, can be achieved by improving the quality of the epoxy resin used for the molding or by a method for increasing the percentage of the filler in the resin.
The prevention of deformation of the stage is most effectively achieved by the reduction of the percentage of the filler in the resin. On the other hand, the prevention of the package cracks is most effectively achieved by increasing the percentage of the filler in the resin. It should be noted that the alternative approach, to apply work on the stage 13a for preventing the package cracks, is costly and has a drawback in that the die attachment of the chip is difficult to perform.
Hence, there is a problem in that the above-mentioned two demands--that is, the reduction of the percentage of the filler for the purpose of preventing the deformation of the stage, and the increase of the percentage of the filler for the purpose of preventing package cracks--contradict each other, and, therefore, it is impossible to meet both these requirements.